In recent years a phenomenon has been discovered in which certain metal oxide-based materials can have a low resistance state and a high resistance state when a voltage is applied, depending on the resistivity prior to application of the voltage and the magnitude of the applied voltage. Interest has been focused on new memory devices that use this phenomenon. This memory device is referred to as a Resistance Random Access Memory (ReRAM). A 3-dimensional cross-point structure has been proposed for the structure of an actual ReRAM device, in which memory cells are disposed at the intersection points of word lines (WL) and bit lines (BL), from the point of view of large scale integration.
In the 3-dimensional cross-point structure, when a voltage is applied to write data to a given memory cell, a voltage is also applied in an opposite direction to other memory cells which have not been selected. Consequently, it is necessary to provide each memory cell with a variable resistance film and a selection element film. For the selection element film, a silicon film with a pin type diode formed therein may, for example, be used.
In order to realize memory cell miniaturization while maintaining reliability and thereby increase the level of integration in a ReRAM structure of this type of structure, the characteristics of the selection element film are required to be favorable and uniform. Hence, it is necessary to form the silicon film using polysilicon, ensure that an impurity concentration profile in the silicon film is sharp, and ensure that a top surface is flat.